Equipment and techniques for increasing the dynamic range of a projection system

ABSTRACT

Apparatus and techniques for enhancing the dynamic range of electronic projection systems are detailed. Included among the techniques are pre-modulation, luminance compensation, and partial luminance compensation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/215,706 filed on Jul. 3, 2000 and U.K. Application Serial No.00163402 filed on Jul. 3, 2000 and International Application No.PCT/US01/23167 filed on Jul. 3, 2001 and published in English asInternational Publication No. WO 02/03687 A2 on Jan. 10, 2002, theentire contents of which are incorporated herein by this reference.

FIELD OF THE INVENTION

This invention relates generally to projection of images and morespecifically to techniques and equipment for enhancing the dynamic rangeof images projected electronically through, typically, digitalprojectors.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,386,253 to Fielding, incorporated herein in its entiretyby this reference, discusses exemplary projection systems utilizing oneor more spatial light modulators (SLMs). As noted in the Fieldingpatent:

-   -   Spatial light modulator devices include so-called “active        matrix” devices, comprising an array of light modulating        elements, or “light valves,” each of which is controllable by a        control signal (usually an electrical signal) to controllably        reflect or transmit light in accordance with the control signal.        A liquid crystal array is one example of an active matrix        device; another example is the deformable mirror device (DMD)        developed by Texas Instruments . . .        See Fielding, col. I, II. 13–21. Of course, yet other types of        light “engines,” or sources, exist, and various of them may be        used in connection with the inventions described herein.

Regardless of the type of light sources and modulators used, audiencesfrequently desire to see images high in detail and richness and low inobjectionable artifacts. High resolution and image quality in particularfacilitates suspension of disbelief of an audience as to the reality ofthe projected images. Such quality indeed often is an important factorin the overall success of the motion picture viewing experience amongtoday's audiences.

Producing these high-resolution images is not without added cost,however. Imax Corporation, for example, the intended assignee of thisapplication, utilizes not only specialized cameras and projectors, butalso seventy millimeter, fifteen perforation film to increase theresolution and quality of projected images. Conventional electronicprojectors (and especially those utilizing SLMs), by contrast, generallycannot supply equivalent resolution in projected images. As well, suchelectronic projectors frequently fail to furnish the dynamic range andoverall brightness of images provided by large-format films. Theynonetheless may desirably (or necessarily) be employed to displaynon-film-based images such as (but not limited to) computer-generatedgraphics or material captured with electronic cameras.

A DMD is a type of SLM that consists of a two dimensional array ofmirrors. The mirror array is imaged through a projection lens onto ascreen so that each mirror functions as an image pixel. Each mirror canbe electronically controlled to assume two positions, one that reflectsincident light towards the projection lens, this is the “on” state, andanother position that does not reflect incident light towards theprojection lens but directs it instead to for example a beam dump, thisis the “off” state.

The DMD is therefore a binary light modulator. Variable intensity may beproduced by controlling the time that a mirror spends in each state, onor off, and repeatedly cycling each mirror between the on and off statesin a regular pattern according to a series of image frames as isconventional in the display of moving images. By varying the amount oftime each mirror spends in the on state during each frame time thebrightness of each pixel can be controlled. This technique is calledpulse width modulation or PWM.

Using PWM a grayscale can be created with a DMD device. This grayscalecan be controlled by input digital data in the form of a binary code.For example, dividing each frame time into ten time periods of differentlengths can create a 10 bit gray scale. The length of the time periodcorresponding to the least significant bit (LSB) in the address signalfor any particular frame is set at a predetermined value, the durationof the time period corresponding to the next significant bit (LSB+1)being twice as long as that corresponding to the LSB and so on. Thus,the length of the time period corresponding to the most significant bit(MSB) for a 10 bit input signal is 512 times that corresponding to theLSB. This gives a total of 1024 possible gray scale values between fullblack (the DMD mirror remains in the off state for the fill frame time)to full white (the DMD mirror remains in the on state for the full frametime). Provided that the lowest PWM frequency for the MSB is above the“fusion frequency” for the human visual system, each of the PWM cycleswill be integrated and provide the sensation of a continuously variablegrayscale corresponding to the binary value of the input signal. Thistechnique is called binary PWM.

Using binary PWM the output brightness level from each mirror is inproportion to the fraction of time that the mirror is “on” within aframe interval. As a result, the output brightness level B from a singleDMD pixel can be modeled by the following equation:B=(αy+δ)TL=αyTL+δTL.  (1)

In equation (I), L is the incident light intensity from a light source,y is the digital signal with normalized values ranging from 0 to 1 and Tis the time duration of each display frame. The factor α<1 representsthe optical efficiency of a DMD pixel. The maximum output or “whitelevel” from a DMD device is obtained when signal value reaches itsmaximum or y=1, i.e.:B _(w) =B _(y=1)=(α+δ)TL.  (2)

Similarly, the minimum output or “dark level” of the DMD device isreached when y=0, i.e.:B _(b) =B _(y=0) =δTL.  (3)

The ratio of maximum to minimum level determines the contrast ratio of aDMD-based projector. The minimum level is the result of unwanted lightbeing reflected into the projection lens pupil when the mirror is in theoff state. This is caused by several factors including scattering fromthe mirror edges and the structure beneath the mirrors. The sources ofunwanted off state light are combined into the term δ in Equation (1).For a DMD device that truly supports an n-bit dynamic range, its darklevel must be less than the brightness level represented by the leastsignificant bit (LSB) of the digital input signal. In other words, thefollowing relationship must be maintained:

$\begin{matrix}{\delta < {\frac{1}{2^{n} - 1}.}} & (4)\end{matrix}$

The dark level or the lowest light level that can be displayed sets alimit to the amount of detail that can be generated in dark scenes. Inthe case of a DMD projector system, the switching speed of the DMDdevice determines the minimum bit time (LSB). In addition, the darklevel represents the minimum displayable level when the DMD is in thefully off state. Reducing the LSB display brightness below some criticalvalue produces little gain in apparent gray scale bit depth since theincrease in grayscale resolution is masked by the DMD dark level. Thedark level also limits the contrast ratio that the system can display.For typical SLM based projection systems this is between 200:1 and 500:1depending on the optical design.

In order for a viewer to perceive images that have a full range oftones, allowing the richest imagery that is as close to reality aspossible it is necessary to provide varying levels of projection systemcontrast depending on the light levels that are provided by the systemand also depending on the ambient light levels of the surroundingviewing environment. The human visual system has a “simultaneouscontrast range” which refers to the contrast range that a typical humanobserver can see at one time in a given state of adaptation to overallscene brightness. This is normally accepted to be in the range of 200:1.However, the human visual system adapts its simultaneous contrast rangeto a much wider range of total scene brightnesses, amounting to aboutseven decades from the darkest part of the scene to the brightest. It iscommon for an observer to change adaptation over a significant portionof this range as the observer's point of regard in the scene changes.This is exemplified by looking at the exterior of a building in brightsunlight, and then looking into the underground parking lot. A typicalviewer can see both cars in the parking lot and features on the brightbuilding even though the total contrast range in this scene exceeds thesimultaneous contrast range that the viewer can perceive.

The projection system contrast that is required to produce a sensationequivalent to a full range of tones increases as the projection lightlevels decrease, and also increases as the surround becomes darker. In atypical motion picture theatre viewing environment a projection contrastof 1000:1 or higher is needed in order for the viewer to perceive a fullrange of tones equivalent to the viewer's simultaneous contrast range of200:1. In addition, the size of the steps in grayscale that are requiredfor a difference to be perceived varies with brightness. An observer candiscriminate between much smaller steps in grayscale at lower levels ofbrightness than he/she can at a higher brightness level.

When representing grayscales using binary data it is common to refer tothe number of bits in the binary numbers used as the “bit depth.” Agreater number of bits obviously produce finer steps in the gray scale,and up b some threshold, determined in part by the viewing conditions, alarger number of gray scale steps, and therefore a larger number of bitsare desirable. However, as discussed above, there is no value insubdividing the grayscale steps below the smallest step that is justperceivable above the dark level of the projection system.

In International Patent Application WO 94/10675 (incorporated herein inits entirety by reference), there is described a method of increasingthe bit depth of a DMD based display system in which the intensity ofthe light source used to illuminate the DMD is modulated on a binarybasis. However, while extending the normal gray scale bit depth (sincethis binary modulation takes place within a single video frame) it hasno effect on the DMD black level. Also with such lamp modulation, thepower supply has to change output very rapidly and thus imposesdemanding design requirements on the lamp power supply and may alsogenerate a significant amount of electromagnetic interference.

The dark level can be reduced as improvements are made to thearchitectural design of DMDs and other light modulating devices, but itmay not be completely eliminated. Therefore, equipment and techniquesfor decreasing the dark level and thus, increasing the dynamic range ofa SLM projector are desirable.

SUMMARY OF THE INVENTION

The present invention seeks to provide such advancements by addressingdeficiencies of, typically (but not necessarily exclusively) electronic,SLM-employing projectors. It further does so in a more comprehensivemanner than heretofore considered, attempting to create equipment andtechniques capable of providing images of sufficient overall qualitythat they may be used in venues instead of, or in addition to,traditional large-format film projectors without disturbing audienceperception that the viewed images are of high quality. As noted above,this perception is a significant aspect of modern-day viewingexperiences, at times helping determine overall success among the publicof particular motion pictures.

Selected embodiments of the invention may use additional SLMs aspre-modulators to improve the dynamic range of the system. Somepreferred embodiments of the invention employ a global or “single-pixel”pre-modulator (typically an SLM) adapted to improve the dynamic range ofthe entire downstream SLM. In operation, the pre-modulator functions toblock light from the downstream SLM to darken its entire image andenhance the black levels of selected scenes. The downstream SLM wouldretain its full dynamic range capability, but would have as its inputnew illumination levels when appropriate or desired. For scenes that arebright, the pre-modulator need not be activated; in which event normalbrightness levels would be maintained. The pre-modulator thus may beused to adapt the projector to scene brightnesses, matching generallyhow the human visual system functions. In another embodiment, two ormore SLMs are arranged so that there exists a precise one-to-onecorrespondence of their pixels. In yet other embodiments, a separatepre-modulator could be used to darken a selected area (e.g. a quadrant),so that precise one-to-one correspondence of pixels is not requiredbetween a particular pre-modulator and the downstream SLM. In any event,each SLM could be driven independently but in concert so that theirdynamic range capabilities would combine to extend the resulting dynamicrange.

Yet additional features of the present invention include luminancecompensation for selectively increasing the illumination levels providedby the downstream SLMs when, for example, further overall scene contrastis desired. Compensation algorithms may particularly be useful whensingle-pixel pre-modulators are used, as the global pre-modulation theyprovide may occasionally diminish too much the input to the downstreamSLMs. Additional features of the present invention include partialluminance compensation in order to avoid highlight clipping on luminanceand color components.

Additional features of the present invention relate to the method ofproviding a control signal to the pre-modulator in concert with theimage data supplied to the SLM.

Other features and advantages of the present invention will be apparentto those skilled in the relevant art with reference to the remainder ofthe text and drawings of this application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematized diagram of an embodiment of the system of thepresent invention.

FIG. 2 illustrates the concept of an adaptive dynamic range window.

FIG. 3 illustrates an example of the luminance compensation algorithm.

FIG. 4 illustrates an example of partial luminance compensation to avoidhighlight clipping.

FIG. 5 is a schematized diagram of an embodiment using a pre-modulatorfor each color channel.

FIG. 6 is a schematic diagram of another embodiment of a display systemin accordance with the invention.

FIG. 7 illustrates the bit mapping of the address signal to the signalsused to drive the DMD in FIG. 6.

FIG. 8 illustrates circuitry that may be used to implement thepre-modulation indicated in FIG. 6.

FIG. 9 is a schematic diagram of another embodiment of a display systemin accordance with the invention.

FIGS. 10 and 11 are, respectively, a side view and a plan view of amodulator for use in the system of FIG. 9.

FIG. 12 is a schematic illustration of the modulator of FIGS. 10 and 11incorporated in the system of FIG. 9.

FIGS. 13 and 14 illustrate differing alignment of the reflectiveportions of the modulator of FIGS. 10 and 11.

FIG. 15 illustrates an alternative modulator for use in the system ofFIG. 9.

FIG. 16 illustrates an alternative optical arrangement for use with themodulator of FIGS. 10 and 11, or FIG. 15; and

FIG. 17 illustrates a further embodiment in which identification of thepre-modulation to be performed on each frame is encoded in the inputsignal.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematized diagram of an embodiment of the systemof the present invention. As shown in FIG. 1, a light source 1 such as alamp directs a light beam on a pre-modulator 2. The pre-modulator 2shown in FIG. 1 is a single pixel or global pre-modulator such as avariable reflectance mirror or a variable transmittance neutral densityfilter. A multi-pixel addressable SLM device could also be used. WhileFIG. 1 shows a DMD, other spatial light modulator devices known in theart could be used.

The pre-modulator 2 controls the amount of light incident on DMD device3. When the pre-modulator reduces the light incident on the DMD the darklevel of the DMD will be reduced. The pre-modulator acts to vary theintensity of the light incident on the DMD in a discrete or continuousfashion according to the method used to implement it.

The action of the pre-modulator can be modeled by a variable β(β≦1),called the pre-modulation factor. When β is less than 1 the incidentlight onto the DMD is reduced.

The output brightness from a DMD device with a global premodulator cansimply be represented byB _(β) =βB.  (5)

The application of a global pre-modulator does not change the contrastratio of a SLM projector at any given instance, since both white anddark levels are reduced by the same factor β,B_(β,w) =βB _(w),andB _(β,b) =βB _(b)  (6)However, since the global pre-modulator changes the full-scale valueexpressed by the dynamic range of the SLM, it effectively changes thesize of the steps between successive grayscale levels displayed by theSLM. The human visual system varies in sensitivity to grayscale stepchanges as the overall scene brightness changes, so a system that canvary the size of the displayed grayscale steps with scene contentresults in higher perceived image quality.

Additionally, the pre-modulator does increase the dynamic range of thesystem, or its total contrast range, since at the maximum attenuationsetting the pre-modulator reduces the black level, while at the minimumattenuation setting essentially the full white level of the projector ismaintained.

In a motion picture film it is common for some scenes to containpredominantly bright tones, and for some scenes to contain predominantlydark tones. It is these darker scenes where the increased black level ofa conventional SLM projector is most objectionable. In addition, theperceived brightness or darkness of a new scene is affected at a scenetransition by the viewer's state of adaptation. Since motion picturesare viewed as a continuous series of changing scenes, the ability of theprojector to adapt to scene brightness operates as a compliment to theviewer's state of adaptation and creates the sensation of an increasedcontrast range.

Therefore, by controlling the amount of pre-modulation, a SLM projectorcan adapt to scene brightness so that the range of brightness deliveredis optimized for human perception. This technology is called “adaptivedynamic range window”(ADRAW) projection. As previously discussed, ourperception of contrast works like a window sliding within a much largeradaptation range, and the location of that “contrast range window” isdetermined by specific viewing conditions.

The existence of a sliding contrast range window within our visualsystem suggests that the most efficient projection system would be theone that is adaptive to scene brightness and viewing conditions. Forcinematic applications, the viewing condition is well controlled, andviewers adapt only to changes in scene brightness. Since viewers onlyperceive a limited range of brightness levels within a certain contrastrange window, it is sufficient for a projector to provide a contrastrange that matches this window while moving the window upward anddownward with each change of scene brightness.

FIG. 2 illustrates the concept of the adaptive dynamic range window(ADRAW) projector. As shown in FIG. 2, the dynamic range window 10 movesup and down the brightness range scale 11 shown as a dotted line.

The global pre-modulator 2 as described with respect to FIG. 1 providesa simple method of implementing the ADRAW concept for a SLM projector.If a scene is bright, the pre-modulator is at the minimum attenuationsetting (β=1), and the projection system outputs maximum brightness. Inthat case, the dynamic range window 10 of the system is positioned atthe top 12 of the scale 11. When a scene is dark, the pre-modulatorattenuates the light (β<1), and the output brightness is reduced, and sois the dark level. When the pre-modulator is at the maximum attenuationsetting, the dynamic range window 10 moves down the scale 11 to thebottom 14. Without pre-modulation, the dynamic range window 10 extendsonly to position 13 on the scale 11. As a result, an ADRAW projectorsystem is capable of supporting a much larger dynamic range than thecontrast range of a conventional SLM projector.

The current window 10 position is controlled by pre-modulation factor β.The value of β is selected based on scene brightness, and it can vary ina continuous range or in discrete steps. The value of β may bedetermined by various techniques such as a thresholding of pixel valuesin the image, averaging of pixel values in the image or other techniquesthat relate to the brightness of the image as perceived by the viewer.

FIG. 2 shows the expanded dynamic range from an adaptive projector withglobal pre-modulation. Assuming the pre-modulation factor varies withinthe following range:β_(min)≦β≦β_(max).  (7)Compared with a DMD system without pre-modulation, the system “white”level is:B _(β,w)=β_(max) B _(w)  (8)and remains unchanged if β_(max)=1, but the system dark level isreduced:B _(β,b)=β_(min) B _(b)  (9)The actual “white” and “black” levels delivered by the projector aredetermined by the amount of pre-modulation, and they can be calculatedfrom equation (6).Data Formatting

The image data that represents the motion picture is supplemented withdata that represents the pre-modulation factor β and as will be shownpossibly other compensation information as well. All of this data may beincorporated in a data stream supplied to the projector. For example, ifthe high definition video standard SMPTE 274M is used, the informationneeded to control the pre-modulator may be formatted into the blankinginterval of this signal. Alternatively, the serial digital videostandard SMPTE 292 may be used and the information encoded into“metadata” in the serial digital video signal according to extensions tothe SMPTE 292 standard currently under consideration. It is alsopossible to use the “alpha” channel to carry the pre-modulation factorwhen using image formats which support it such as the dual link 4:4:4:4mode of SMPTE 292.

While it is preferable for each scene to be viewed under representativeconditions with a pre-modulator equipped projector so that thepre-modulation factor and other compensation information can bedetermined interactively by a skilled operator, it is also possible foran automatic selection of the pre-modulation factor and othercompensation to be made by circuitry in the projector. This circuitrywould consider scene brightness and according to pre-determined rulesselect the appropriate pre-modulation factor. Other hardware between theinput data and the SLM would modify the pixel values as required toimplement any additional compensation as described next.

Luminance Compensation

Simply sliding a dynamic range window up and down the brightness scaledoes not always deliver the best visual results. For example, in a test,global pro modulation was applied to a relatively dark scene and thepre-modulation factor β was set to 0.5. Although image brightness wasreduced by 50%, the contrast ratio should have remained unchangedbecause the dark level was also reduced by the same factor. However, ascan be predicted from data on the human visual system there was a lossof apparent contrast due to the reduced overall brightness of the image.Furthermore, colors appeared less saturated when total scene brightnesswas reduced.

To improve perceived image quality, the apparent loss of contrast can becompensated through a luminance compensation algorithm. Tile purpose ofthe algorithm is to modify the pixel code values so that the overallapparent image contrast is retained.

FIG. 3 illustrates the effect of pre-modulation on the gray scaletransfer function of the projector. The relationship between theluminance Y and the pixel code values p for a projector beforemodulation 20 and after modulation 21 is shown. Assuming Y is somefunction of p (the characteristic curve of the projector), therelationship between luminance Y and pixel code p, is shown by:Y=f(p)  (10)then pre-modulation reduces the output luminance by:Y=f _(β)(p)=βf(p).  (11)

In FIG. 3 dotted line 22 shows the maximum luminance Y=50% that is themaximum white level determined by β=0.5. As this figure shows,brightness is reduced. Luminance compensation is introduced to increasethe contrast by multiplying each pixel code by a compensation factor c,so that:f _(β)(cp)=f(p).  (12)The amount of compensation c for each pixel code can be calculated fromequation (12), which results in:

$\begin{matrix}{c = {\frac{f_{\beta}^{- 1}( {f(p)} )}{p} = {\frac{1}{p}{{f^{- 1}( \frac{f(p)}{\beta} )}.}}}} & (13)\end{matrix}$FIG. 4 shows how luminance compensation operates. Here the projectortransfer function without pre-modulation 30 is shown along with thepre-modulated function 31. The fully compensated transfer function 33has the same slope in the mid-tone and highlight areas until the maximumvalue of Y=50% is reached.Highlight Clipping and Partial Compensation

The luminance compensation algorithm improves visual quality, but it mayalso introduce highlight clipping. Highlight clipping occurs when thecompensated code value required for full compensation exceeds themaximal value of an n-bit pixel code p,

$\begin{matrix}{{cp} = {{f^{- 1}( \frac{f(p)}{\beta} )} > {2^{n} - 1}}} & (14)\end{matrix}$In other words, the compensation code c cannot bring the pixel code p tothe appropriate level. Highlight clipping can be seen in FIG. 4 wherethe compensated transfer function 33 is clipped where it exceeds themaximum brightness possible as defined by line 32 which is determined bythe pre-modulation factor β=0.5. For all pixel values p greater than thepixel values corresponding to the point of intersection of thecompensated transfer function 33 with line 32, clipping will occur sinceall of these greater pixel values will result in the same luminancevalue Y=50%.

Clipping in image highlights removes details in highlight regions andintroduces visible degradation to the image. It is common to balance thewhite point of a RGB color SLM based display by adjusting the gains ofthe three color channels. A color shift in the highlight areas may occurif clipping occurs on only one or two color channels because of thedifferent gains in the three channels. For instance, a blue sky mayappear yellowish after clipping, since the saturated blue channel willbe the one most affected by clipping.

Highlight clipping can be avoided if the amount of pre-modulation isselected so that it covers the entire luminance range of the originalimage:f _(β)(2^(n)−1)≦f(p _(max))  (15)where p_(max) is the maximum pixel value in the original image data.However, the amount of pre-modulation that satisfies condition (15) maynot provide the desired black level for a given scene. In addition,simpler pre-modulation schemes may only allow certain discrete valuesfor β. There will always be a trade-off between enhanced black level andhighlight clipping.

For images where the desired black level is not obtained when equation(15) is satisfied, partial compensation can be applied to reducehighlight clipping. Partial compensation is the result of relaxing thefull compensation condition (12) to bef _(β)(cp)<f(p).  (16)

In FIG. 4, when pre-modulation factor β=0.5 is used to obtain transferfunction 31, highlight clipping occurs when full compensation is used asshown by transfer function 33. Partial compensation (c′<c) may beapplied as shown in transfer function 34, where the luminance Y aftercompensation will be f_(β1)(c′ p) which is not clipped. It is alsodesirable to apply a shoulder or “soft clip” such as shown at 35 tocompensated transfer functions where clipping occurs so that thecompression of the highlights is more gradual.

It is possible to employ a variety of compensation schemes to the imagedata, and to set the points at which full or partial compensation iseffective depending on the scene content. This is best accomplished byviewing the scene and setting the compensation and pre-modulationvalues. Compensated pixel values for each frame in each scene are thenobtained by image processing. A hardware look-up table may also beemployed to supply the compensated pixel data. In this case the look-uptable is addressed by the input data and the look-up table entries aremodified by the pre-modulation settings and the compensation factor sothat the look-up table outputs are the compensated pixel values requiredfor each pre-modulator setting.

It is also essential that the projector characteristic curve be known.This is best obtained through measurement under actual projectionconditions. Such a measurement will map input pixel values to outputbrightnesses while taking into account all of the non-linearities in thesystem due to image data formats and transfer functions that may beinherent in the data such as gamma correction in video images or thegamma of images scanned from film.

In one embodiment, the luminance off the screen is measured using aMinolta CS-100 chroma meter. The measurement is taken at the sameposition near the center of the screen. A 17-step full frame grayscalewedge is used for the entire data range and a 32-step grayscale wedge isused for the lower luminance values. Each grayscale wedge is projectedonto a high-gain screen and the corresponding CIE Y component measuredfrom a fixed position near the center of the screen. The data is thenfit with a piece-wise curve:

$\begin{matrix}{Y = {{f(p)} = \{ \begin{matrix}{{{( \frac{p}{2^{n} - 1} )^{\gamma 2}( {Y_{h\;\max} - Y_{h\;\min}} )} + Y_{h\;\min}},} & {64 \leq p \leq 255} \\{{{( \frac{p}{2^{n} - 1} )^{\gamma 1}( {Y_{1\;\max} - Y_{1\;\min}} )} + Y_{1\;\min}},} & {0 \leq p < 64}\end{matrix} }} & (17)\end{matrix}$The gamma values γ1 and γ2 for both the top part and lower part of thecurve are determined by curve fitting.

While this embodiment is described with a light source used toilluminate a single DMD, it will be appreciated that usually severalDMDs will be incorporated, these being included in separate colorchannels. Furthermore, each color channel may itself include more thanone DMD. Spatial light modulators other than DMDs may also be used, asis known by one skilled in the art.

It should also be clear to those skilled in the art that if apre-modulator is available with more than a single “pixel” or a globaleffect, pre-modulation may be selectively applied to parts of the image,using the same principles described above. The principles describedremain effective up to and including a pre-modulator implemented usinganother SLM where there is a one to one correspondence between thepixels of the two SLM devices.

Pre-Modulation On Luminance and Color Components

In the embodiments described above, a single pre-modulator is appliedafter the light source and before the DMD prisms. Therefore, theanalysis focused on luminance component Y, which can be either measuredby a chroma meter or be calculated from RGB values of the image data.For image data that is captured by a video camera or processed for videodisplay, the RGB components are nonlinear, and the standardizeddefinition recommended by ITU-R 601 can be used:Y=0.299R+0.587G+0.114B  (18)For linear RGB data, the luminance definition recommended by ITU-R 709should be applied:Y=0.2125R+0.7154G+0.0721B  (19)

All luminance compensation is done based on the calculated luminancevalue. The problem with a single pre-modulator is that highlightclipping can result in color shifting, as discussed above.

One solution is to have a pre-modulator in each color channel, and theamount of pre-modulation in each channel is controlled by the luminanceof each individual color. This embodiment is illustrated in FIG. 5. Asshown in FIG. 5, pre-modulator 40 is used for the red channel,pre-modulator 41 is used for the green channel, and pre-modulator 42 isused for the blue channel.

All previous mathematical analysis applies if total luminance isreplaced by the luminance of individual color channels. It is possiblefor the characteristic curves of individual color channels to bedifferent, and this must be taken into consideration. The amount ofpre-modulation for individual color channels can be different from theother channels, so that better control of color balance can be achieved.This configuration also allows black level color balance.

Alternative Embodiment

In the previous embodiment, the pre-modulator operates in concert withthe SLM, with both active on every frame, with the resulting intensityof each image being the product of the intensity selected by thepre-modulator and the intensity selected by each pixel of the SLM. In analternative embodiment, the setting of the pre-modulator is establishedfor a number of frames, typically a scene, and changes in pre-modulationsetting occur at scene changes.

It is possible to take advantage of the integrating action of the humanvisual system and perform pre-modulation across two or more frames,relying on the integration of the different intensities in each frame toproduce an extended gray scale. This aspect of the invention will now beexplained.

Referring first to FIG. 6, this diagram shows schematically a displaysystem including a projector system including a DMD 101, a lamp 103 anda lens 106, the projector system being arranged to project an image ontoa cinema screen 109. The DMD 101 is illuminated by the lamp source 103,which is powered by a power supply unit 105. The DMD typically has anarray of 1280×1024 mirror elements, the orientation of each of which iscontrolled by data signals supplied to the DMD. The lamp will typicallybe a high power Xenon arc lamp. The spatially modulated light producedby the DMD in response to the data signal is focused by the lens 107onto the cinema screen 109.

The arrangement as described above is a conventional projector system.However, in accordance with the one embodiment, the light which isdirected onto the DMD 101 is pre-modulated by modulating the currentsupply to the arc lamp via power supply unit 105 such that the lamp 103has an N-bit modulated output.

If the lamp brightness is changed by an amount equivalent to a binaryamount N, then it can be shown that the DMD black level is reduced by anamount 2^(N). As will be explained in more detail hereafter, thispre-modulation is applied selectively to groups of frames such as a pairof successive frames K1, K2, by determining whether the imagecorresponding to a particular frame of the data signal has brightnessvalues corresponding to a bright scene (Mode 1), a dark scene (Mode 2)or an intermediate light level scene (Mode 3).

The mapping of the bits displayed by the mirror elements of the DMD isalso modified to correspond to the operating mode.

In the particular embodiment being described, the electronics for theprojector has to determine the required pre-modulation mode and set theprojector up accordingly.

In its simplest form, if an input MSB is present then Mode 1 isrequired. If neither the MSB nor the MSB-1 is active, then Mode 2 isrequired. If the MSB is inactive and the MSB-1 is active, then Mode 3 isrequired. Then, in Mode 1 as will be explained in more detail hereafter,a bit mapper will route the top M input bits to K1 and K2. Similarly inMode 2, the bit mapper will route the bottom M bits. In Mode 3, the bitmapper will route the top “α” bits starting from the input MSB-1 to K1and the bottom (M+N−α) bits including the virtual bit to the bottom bitsof K2.

However, the situation could arise in which just one pixel has the MSBset (or the MSB-1 “ON” and the MSB “OFF”). Under these conditions theabove detection algorithm would force a Mode 1 (Mode 3) operationbecause of that one pixel when image scene content would suggest a Mode3 (Mode 2) as being more optimum.

One option is to count the number of active bits in the MSB and theMSB-1 bit planes. The trigger points for mode selection would then beinitiated if the number of active pixels exceeds some threshold. Thiswould be fine but for all those pixels for which the MSB is active.Since this is discarded in Mode 3, then all these pixels would displayan erroneous gray scale level. The solution to this is to force thesepixels into saturation. This is achieved by setting all the input bitsto logical 1 when Mode 3 and the input MSB are both active. A similararrangement can be applied to the MSB-1 during Mode 2 operation.

This can be extended to take into account the number of adjacent pixelscontributing to the sum thereby taking into account the effect of areahighlights. In fact the threshold algorithm can be extended to take intoaccount as many scene situations as is deemed necessary when designingthe pre-modulator. The sums are stored with each frame of image data asthe incoming data is processed. These sum values are then compared withthe threshold values when displaying that frame of image data and thepre-modulation mode set up accordingly.

In the following example, in order to simplify the explanation, it isassumed that the image data signal has nine bits after degammacorrection and that the DMD is capable of 6 bit resolution (i.e. M=6)rather than the usual 10 bit resolution, and the pre-modulation providesa 2 bit modulation depth (i.e. N=2). These bits are shown as the centralsequence of bits in each of FIGS. 7( a), 7(b) and 7(c). The left-handbit in each bit sequence corresponds to the MSB. The right-hand bit ineach bit sequence corresponds to a very low light level corresponding tothe LSB/2, that is, a low light level that cannot normally be displayedby the DMD. Accordingly, this bit is shown dotted.

The bit sequences for each frame K1 and K2 in FIGS. 7( a) and 7(b) eachhave 6 bits (the maximum that can be supported in this particularexample) corresponding to M=6, the bit resolution of the DMD 101. InMode 1 illustrated in FIG. 7( a), the intensity of the light incident onthe DMD 101 is not attenuated. In the case of a bright scene, the DMDfor both frames K1 and K2 is arranged to display the six uppermost bits,that is, the bits corresponding to the highest light levels. In Mode 2,the lamp current from the power supply unit is reduced so as toattenuate the light output of the lamp by 2^(N) for the lowest lightlevels. As illustrated in FIG. 7( b), the DMD in both frames K1 and K2is arranged to display the lowest six displayable bits, but excludingthe LSB/2 bit (which the DMD cannot display).

As explained above, even if the image is generally dark, there may existpixels that should be switched “ON” so as to display the MSB or MSB-Ibits, that is the left hand bits. In this case, in Mode 2 it is arrangedthat if the MSB or MSB-1 should be switched “ON”, in displaying thesebits the next six lowest bits are all switched “ON”. It will beappreciated, however, that this will slightly reduce the brightness forpixels in which the MSB should be switched “ON”.

Finally, as illustrated in FIG. 7( c), where it is determined that theimage has a generally intermediate luminance value, the lamp is switchedbetween brightness levels on alternate frames with K2 corresponding tothe low brightness situation. Thus, the output of the lamp is notattenuated in frame K1, but is attenuated in frame K2. K1 is loaded withthe top M bits below the MSB (i.e. ×2 multiplication and the LSBdiscarded) when the lamp intensity is high and K2 is loaded with thebottom M bits including the LSB/2 bit when the lamp intensity is low.

In the first frame K1 a chosen number of bits (in this particular case,a bits where a is 4) are chosen to be displayed. The value of α isdetermined so as to give satisfactory results for any particular imagesignal. For frame K1 the MSB is never displayed, but as in Mode 2 wherethe MSB for a particular pixel should have been “ON”, all the remainingbits are arranged to be “ON” instead. Thus, in K1, the high light levelsare displayed.

In the second frame K2, however, the lowest four bit levels aredisplayed. In this particular case, although the extra bit LSB/2 cannotbe normally displayed, by display of a LSB on every other frame (i.e.during K2 only) means that LSB/2 can be displayed. The eye will, ofcourse, be effective to integrate the light level over K1 and K2 to getthis extra bit resolution.

Turning now to FIG. 8, this figure illustrates an example of thecircuitry for implementing the pre-modulation of the light incident onthe DMD 101 as explained in relation to FIGS. 6 and 7. In FIG. 8,equivalent elements to those in FIG. 6 are correspondingly labeled.

The image input signal which, in the particular example to be describedis a seven bit digital signal representative of successive frames of amovie film including frame synchronization signals and line signals, isinput into a degamma circuit 301. This degamma circuit is effective toremove the gamma modulation on the input signal which has been includedto match the form of the image signal for display on a display device,such as a cathode ray tube which has a non-linear transfercharacteristic. It will be appreciated that the removal of the gammamodulation is necessary to match the signal to the linear transfercharacteristic of the DMD 101. This is explained in U.S. Pat. No.6,034,660 incorporated herein in its entirety by reference.

Due to the degamma operation, the degamma circuitry is effective to add,in this particular example, two further bits to the input image signal.The nine bit output of the degamma circuitry 301 corresponds to theinput signal of M+N+1 bits illustrated in FIG. 7. This is input to aformatter 303 which is effective to produce bit planes of the bits LSB,LSB+1 . . . , MSB-1, MSB of the “ON” times for each pixel of each frameof the image signal as explained in more detail in, for example, U.S.Pat. No. 5,673,060, the contents of which are hereby incorporated byreference. The output of the formatter 303 is applied to a double framestore 305, 307 arranged such that one half of the double frame store canbe loaded at the same time as the other half of the double frame storeis unloaded. Suitable switching means (not shown) for switching betweenthe two parts of the frame store 305, 307 will be incorporated.

As so far described the address system for the DMD 101 is conventional.However, the projector system also includes circuitry for determiningwhich of the three modes, Mode 1, Mode 2, Mode 3 described above isgoing to be used to display each pair of frames K1, K2 as will now bedescribed.

Outputs from the formatter 303 are also connected to an active bit sumweight generator 309 which looks at the top N (N=2 in this case) bits todetermine how many pixels within a frame the top two bits MSB, MSB-1 areswitched “ON” and which pixels these are. The output of the sumgenerator 309 is applied to a sum data double store 311, 313 which isagain arranged such that one store is being written while the other datastore is being read out, suitable switching means again not being shownin the figure.

The output of the relevant data store 311 or 313 is applied to a decoder315 which is effective to compare the number of pixels in which the MSBor MSB-1 is switched “ON” with a threshold value to determine whetherthe frame is a relatively high luminance frame (Mode 1), low luminanceframe (Mode 2) or an intermediate frame (Mode 3). An output of thedecoder 315 is arranged to provide appropriate control signals to thelamp power supply 105 to cause the current to the lamp 103 to vary suchthat the lamp either provides an unattenuated (Mode 1 and frame K1 inMode 3) or attenuated (Mode 2 and frame K2 in Mode 3) output.Appropriate signals are also applied by the decoder 315 to a unit 316effective to map the data bits from the frame store 305 or 307 and applythe values to a selector 321 which is effective to select either the bitvalues for K1 or the bit values for K2. It will be appreciated that, inthe case of Modes 1 and 2, the values for the flames K1 and K2 will beidentical. However, in the case of Mode 3, the bit patterns illustratedin FIG. 7( c) are used in which the top four bits are applied in frameK1, while the lowest four bits including the LSB/2 bit is applied inframe K2. These values are used to address the DMD to switch selectedpixels “ON” within each bit plane.

In order to avoid errors in image brightness caused by the MSB or MSB-1light not being displayed in bright areas of the image when Modes 2 or 3are selected, the projector system includes a gate 319 which, when suchpixels are identified, enables circuit 319 to cause all the MSB-2 to LSBinput bits in Mode 2 or all the MSB-1 to LSB/2 input bits in Mode 3 setto the active state for the identified pixels.

It will be appreciated from the above that each frame is effectivelydisplayed twice with the same values being displayed in the cases ofModes 1 and 2 and different values in Mode 3, the human eye beingeffective to integrate the light from the two different light levelsover the successive frames to form an average value. The incoming imagesignal will often have been derived from a 24 frames per second input,this being doubled to display 48 frames per second, each frame beingrepeated twice. However, in some circumstances it may be possible toapply the same principles to different frames.

Turning now to FIG. 9, pre-modulation of the light intensity of thelight incident on the DMD can also be achieved by including apre-modulator 401 between the lamp 103 and the DMD 101. In this case thecontrol signals shown in FIG. 8 ES being supplied to the lamp powersupply 105 are supplied instead to the pre-modulator 401.

The pre-modulator may take the form of a mechanical device for effectingthe transmission of the light from the lamp or, alternatively, may be anelectrical device.

Referring now to FIGS. 10 to 14, an example of a mechanicalpre-modulator 401 takes the form of two circular plates 601, 603. Eachplate is formed with a series of trapezoidal mirrored spokes 605 asshown in FIG. 11, the remaining portions 607 of the plates beingtransparent. The plates 601, 603 are arranged in a stack as indicated inFIG. 10 with the front plate 601 remaining static and the back plate 603being rotated by a stepper motor 609 under the control of a motorcontrol unit 610.

Referring now particularly to FIG. 12, this figure illustrates thepre-modulator inserted in the light path between the lamp 103 and theDMD 101. The rotation of the stepper motor 609 is controlled by thepre-modulation data supplied to the motor control unit 610 such that inthe configuration shown in FIG. 13, the mirrored portions 607 on each ofthe disks 601, 603 are aligned, this resulting in 50% attenuation of thelight from the lamp 103 as light which is not intercepted by themirrored fingers 605 will pass straight through to be detected byphotodetector 611. At the extreme as illustrated in FIG. 14 where themirror portions 605 are totally out of alignment, light which is notintercepted by the front set of mirrored portions will be intercepted bythe mirror portions on the rear plate 603, thus giving minimumattenuation of the light. It will be appreciated that intermediatepositions will give varying amounts of attenuation. The output of thephotodetector is supplied to the motor control unit 610 to synchronizethe stepper motor to produce the required attenuation.

It will be appreciated that it is quite common in a DMD projectionsystem for the light from the lamp to undergo a 90° rotation by a mirrorangled at 45°, this may include a cold mirror for infra-red radiationremoval. Accordingly, a cold mirror can be incorporated in this rotatingshutter approach in order to simultaneously remove unwanted infraredwavelengths from the light emitted from the lamp 103.

As an alternative, the twin disc mechanical pre-modulator of FIG. 12could be replaced by an electronic version comprising an LCD reflectiveshutter 701 with silvered stripes 703 on the active face as illustratedin FIG. 15. Such an arrangement will avoid the necessity of mechanicallyaligned parts and enable a wider range of attenuation levels to beachieved.

FIG. 16 shows an alternative optical arrangement including a mirroredprism 705 interposed in the light path between the lamp 103 and the DMD101. The modulator is as shown in FIG. 15, i.e. a liquid crystal panel701, typically a ferroelectric liquid crystal, with a silvered grating703 overlying the liquid crystal. In use of the pre-modulator, the faceof the prism 706 opposing the lamp 103 directs the incident light ontothe liquid crystal panel 701. Where the liquid crystal panel 701 iselectrically switched so as to be reflective, maximum light reflectancefrom the liquid crystal and the overlying grating 703 will occur withthe light beam then being reflected from the surface 707 of the prism705 towards the DMD 101. In analogous manner to the mechanicalpre-modulator where the liquid crystal panel 701 is switched so as to benon-reflective, the light will be attenuated to an extent determined bythe widths of the reflective portions of the grating 703. It will beappreciated that the prism of FIG. 16 may also be used with a mechanicalmodular as in FIGS. 10 and 11.

It will be appreciated that the use of the prism 705 enables the inputand output light for the pre-modulator to be on the same axis.

In the above embodiment, the decision as to whether each pair of framesshould be Mode 1, Mode 2 or Mode 3 is made automatically in theprojector. However, it is possible for the input image signal to beencoded with instructions as to which mode each pair of frames shouldbe. This coding would typically be performed by the colorist oninspection of the color values or the luminance of the frames. In thesecircumstances, the circuitry shown in FIG. 8 may be simplified as shownin FIG. 17. The decoder 315, data bit mapper 317, gate 318, “Force allON” circuitry 319, and frame K1 or K2 selector 321 are omitted, and apre-modulator data extract block 801 added in the input signal path.This block 801 is effective to remove the instruction code, and provideappropriate signals to a pre-modulator double data store, which iseffective to store the appropriate pre-modulation instructions forcontrolling the lamp power supply 105. The bit data input into thedouble data stores 305, 307 will be pre-encoded with the relevantmodified bit sequences.

It will be appreciated that, while the circuitry for adjusting the lamppower supply is modified in FIG. 17, the circuitry for pre-modulatingthe light 401 can similarly be modified.

It will be appreciated that in order to simplify the explanation of theabove alternate embodiments are described with respect to a single colorchannel, the embodiments being equally relevant to projection systemsincluding one or more DMDs in each of three parallel color channels forred, blue and green light split off from the lamp 103, the image signalincluding R, G, B signals or alternately luminance and chrominance data.Each color channel will normally undergo the same pre-modulation, thecriteria for which of the three modes is used being determined from theR, G, B values or the luminance values. Thus, for example, where onecolor channel R, G, B includes high levels of MSBs, the same mode willbe selected in all three color channels. The invention is alsoapplicable to a serial color system using, for example, a color wheel tosequentially project red, green and blue light onto one or more DMDs.

It will be appreciated that the invention is applicable to systems inwhich the input image data signal is a video signal or a signal otherthan a video signal, which has been produced specifically for use with aspatial light modulator.

It will also be appreciated that the invention is applicable to spatiallight modulators other than DMDs, for example, liquid crystal devices,in particular, ferroelectric liquid crystal devices having fastswitching times.

While the embodiments have been explained on the basis of gray scaleproduced by a pulse-width-modulation technique, with bit planes for theMSB down to LSB being produced in the bit frame stores, the invention isalso applicable to modified bit sequences, for example including lowlight level spatial dithering, as described in U.S. Pat. No. 6,064,366,fractional bits as described in U.S. Pat. No. 5,686,939, bit splittingto improve the temporal balance in each frame as described in U.S. Pat.No. 5,986,640, and bit stuffing as described in U.S. Pat. No. 6,057,816(corresponding to EP 0755556), the contents of each of these patentsbeing incorporated herein in their entirety by reference.

It will be appreciated that, while in the alternate embodiments theframes are grouped into pairs of frames, the invention is applicable toprojection systems in which the groups comprises three or more frames.

It will also be appreciated that, while the light intensity is modulatedto two different intensities (i.e. N=2), it is possible to increase theresolution further by modulating the light to three or more differentintensities.

Because the foregoing is provided for purposes of illustrating,explaining, and describing embodiments of the present invention, furthermodifications and adaptations to these embodiments will be apparent tothose skilled in the art and may be made without departing from thescope or spirit of the invention. Yet additionally, ferroelectricdevices, liquid-crystal displays (LCD), or other light sources or valvesor filters may be employed as necessary or desired.

1. A method of decreasing the dark level of a frame, the methodcomprising: receiving pixel code values for the frame; determining thebrightness of the frame; determining a pre-modulation factor based onthe brightness of the frame, wherein the pre-modulation factor is usedto vary, between at least two different intensities, the intensity oflight incident on a spatial light modulator of a projection system; andmodifying pixel code values for the frame so that the brightness of eachpixel remains substantially unchanged while the dark level of the frameis reduced.
 2. The method of claim 1, further comprising: receiving adata stream containing the pre-modulation factor and modified pixel codevalues by a projection system having a spatial light modulator.
 3. Themethod of claim 1, wherein the pre-modulation factor is selected by theprojection system and the pixel code values are modified by theprojection system.
 4. The method of claim 1, wherein the pre-modulationfactor is determined by a thresholding of pixel code values in theframe.
 5. The method of claim 1, wherein the pre-modulation factor isdetermined by averaging the pixel code values in the frame.
 6. Themethod of claim 1, wherein the pre-modulation factor is chosen to coveran entire luminance range in the frame.
 7. The method of claim 1,wherein the pre-modulation factor is used to control the output of alight source.
 8. The method of claim 1, wherein the pre-modulationfactor is used to control a second spatial light modulator.
 9. Themethod of claim 1, wherein the pixel code values are modified bymultiplying each pixel code value by a compensation factor.
 10. Themethod of claim 1, wherein pixel code values are modified by a variableamount depending on both the pixel code value and the amount of lightincident on the spatial light modulator.
 11. A method of decreasing thedark level of a flame, the method comprising: receiving pixel codevalues for the frame; determining the brightness of the frame;determining a pre-modulation factor based on the brightness of theframe, wherein the pre-modulation factor is used to vary, between atleast two different intensities, the intensity of light incident on aspatial light modulator of a projection system; and partially modifyingpixel code values for each frame so that the brightness of each pixelremains substantially unchanged while the dark level and highlightclipping of the frame is reduced.
 12. The method of claim 11, whereinthe pixel code values are partially modified so that they make a gradualtransition into clipping.
 13. The method of claim 11, wherein the pixelcode values are partially modified by multiplying each pixel code valueby a compensation factor so that a maximum brightness for each pixel isnot exceeded.
 14. A projection system including: a spatial lightmodulator comprising an array of switchable elements, each switchableelement being switchable to an “ON” state in which light incident on thearray is directed to a projection lens in response to data signals inthe form of bits representing successive frames of an image data signal,the spatial light modulator producing gray scale images representativeof each frame by pulse-width-modulation of the “ON” times; means forvarying between two or more different intensities, the intensity oflight incident on the spatial light modulator in successive groups offrames dependent on the brightness values of each group of frames; andmeans for adjusting the bits to be displayed in each group of framesdependent on the intensity of light incident on the spatial lightmodulator.
 15. A projection system according to claim 14, in which saidmeans for modulating comprises means for adjusting the power supplied toa light source effective to illuminate the spatial light modulator. 16.A projection system according to claim 14, in which the means formodulating comprises a variable attenuation device in the light path tothe spatial light modulator.
 17. A projection system according to claim16, hi which the means for modulating comprises two reflective areas,and means for varying the reflectance of one of the areas.
 18. Aprojection system according to claim 16, in which the two reflectiveareas are mutually movable, and the means for varying is effective tomove the areas relative to each other such that said one area is maskedby the other area so as to reduce the overall reflectance.
 19. Aprojection system according to claim 17, in which the reflective areawith the switchable reflectance is a liquid crystal panel.
 20. Aprojection system according to claim 14, in which the spatial lightmodulator is a DMD.
 21. A projection system according to claim 14, inwhich the spatial light modulator is a LCD.
 22. A projection systemaccording to claim 14 in which each group comprises two frames.
 23. Aprojection system according to claim 22, in which the means formodulating is arranged to modulate between two different lightintensities, the intensity of adjacent pairs of frames either beingattenuated, not attenuated or one frame attenuated and the other framenot attenuated.
 24. A projection system according to claim 14, in whichthe projector system includes means for determining the light modulationdependent on the number of pixels in each frame which have more or lessthan a threshold brightness.
 25. A projection system according to claim14, in which the image data signal includes codes for determining thelight modulation for each frame, and the projector system includes meansresponsive to said code to control the light modulation.
 26. Aprojection system according to claim 14, further comprising a lightsource effective to illuminate the spatial light modulator.
 27. A methodof increasing the dynamic range of a projection system including aspatial light modulator comprising an array of switchable elements, themethod comprising: varying, between at least two different intensities,the intensity of light incident oil the spatial light modulator insuccessive groups of frames dependent on the brightness values of eachgroup of frames; adjusting signals in the form of bits to be displayedin each group of frames dependent on the intensity of light incident onthe spatial light modulator; and switching an element of the spatiallight modulator to an “ON” state in response to data signals such thatlight produced from a light source is directed to a projection lens. 28.A projection system including: a spatial light modulator comprising anarray of switchable elements, each switchable element being switchableto an “ON” state in which light incident on the array is directed to aprojection lens in response to data signals in the form of bitsrepresenting successive frames of an image data signal, the spatiallight modulator producing gray scale images representative of each frameby pulse-width-modulation of the “ON” times; means for varying betweentwo or more different intensities, the intensity of light incident onthe spatial light modulator in adjacent groups of frames, wherein thegroup of frames consists of at least two frames, dependent on thebrightness values of each group of frames; and means for adjusting thebits to be displayed in each group of frames dependent on the intensityof light incident on the spatial light modulator.